Inductive power coupling, as known in the art, allows energy to be transferred from a power supply to an electric load without connecting wires. A power supply is wired to a primary coil and an oscillating electric potential is applied across the primary coil, thereby inducing an oscillating magnetic field. The oscillating magnetic field may induce an oscillating electrical current in a secondary coil placed close to the primary coil. In this way, electrical energy may be transmitted from the primary coil to the secondary coil by electromagnetic induction without the two coils being conductively connected. When electrical energy is transferred from a primary coil to a secondary coil the coil pair are said to be inductively coupled. An electric load wired in series with such a secondary coil may draw energy from the power source wired to the primary coil when the secondary coil is inductively coupled thereto.
Induction type power outlets may be preferred to the more common conductive power sockets because they provide seamless power transmission and minimize the need for trailing wires.
Low power inductive electrical power transmission systems have been proposed. One such example is described in U.S. Pat. No. 7,164,255 to Hui. In Hui's system a planar inductive battery charging arrangement enables electronic devices to be charged. The system includes a planar charging module having a charging surface on which a device to be charged is placed. Within the charging module, and parallel to the charging surface, at least one, and preferably an array of primary windings are provided. The primary windings inductively couple with secondary windings within the device to be charged.
Such systems provide inductive coupling at relatively low power adequate for charging batteries. It will be appreciated however, that base units such as Hui's charging surface which transmit energy continuously, in a largely uniform manner over an extended area, are not suitable for use with high energy systems, such as those required to power computers, light bulbs, televisions and the like.
Energy losses associated with high power inductive transfer systems are typically larger than those in low power systems such as Hui's charging surface. In addition whereas in low power systems excess heat may be readily dissipated, an uncoupled high power primary coil or its surroundings may become dangerously hot.
Moreover, the oscillating voltage in a high power primary coil produces an oscillating magnetic field. Where a secondary coil is inductively coupled to the primary coil, the resulting flux linkage causes power to be drawn by the secondary coil. Where there is no secondary coil to draw the power, the oscillating magnetic field causes high energy electromagnetic waves to be radiated in all directions which may have undesired side effects, such as erasing data from credit cards and may be harmful to bystanders particularly to those with pacemakers.
U.S. Pat. No. 6,803,744, to Sabo, titled “Alignment independent and self-aligning inductive power transfer system” describes an inductive power transfer device for recharging cordless appliances. Sabo's device includes a plurality of inductors which serve as the primary coil of a transformer. The secondary coil of the transformer is arranged within the appliance. When the appliance is positioned proximate to the power transfer device with the respective coils in alignment, power is inductively transferred from the device to the appliance via the transformer.
The inductors of Sabo's system are arranged in an array and connected to a power supply via switches which are selectively operable to activate the respective inductors. These selectively operable switches are provided to conserve power and to eliminate objectionable electromagnetic fields. '744 thus indicates the problem of electromagnetic leakage as well as the need for each primary coil to be energized from the power supply only when a secondary coil is within effective range. Furthermore the power receiving units described in '744 are bulky and impractical for use with small electrical devices.
The need remains therefore for a practical inductive power transfer system for safely and conveniently delivering power wirelessly from inductive power outlets to inductive power receivers in an energy efficient manner. The present invention addresses this need.